Appendix E: X65 Quick Reference Cheat Sheet¶

65816 CPU Registers¶

Register	Size	Description	Notes
A (,B=C)	8/16-bit	Accumulator	Width controlled by m flag (0=16-bit)
X	8/16-bit	X Index Register	Width controlled by x flag (0=16-bit)
Y	8/16-bit	Y Index Register	Width controlled by x flag (0=16-bit)
S	16-bit	Stack Pointer	Points to next free location on stack
D	16-bit	Direct Page Register	Base address for direct page addressing
PBR	8-bit	Program Bank Register	Current code bank (upper 8 bits of PC)
DBR	8-bit	Data Bank Register	Default data bank for absolute addressing
P	8-bit	Processor Status	Contains status flags

Status Register (P) Flags¶

Bit	Flag	Description	When Set (1)	When Clear (0)
7	N	Negative	Result is negative	Result is positive
6	V	Overflow	Overflow occurred	No overflow
5	M	Memory/Accumulator Select	8-bit accumulator	16-bit accumulator
4	X	Index Register Select	8-bit index registers	16-bit index registers
3	D	Decimal Mode	BCD arithmetic mode	Binary arithmetic mode
2	I	IRQ Disable	IRQ interrupts disabled	IRQ interrupts enabled
1	Z	Zero	Result is zero	Result is non-zero
0	C	Carry	Carry occurred	No carry occurred

Common 65816 Instructions¶

Register Instructions¶

REP #$30      ; Reset bits in P (set 16-bit A,X,Y mode)
SEP #$30      ; Set bits in P (set 8-bit A,X,Y mode)
XBA           ; Exchange B and A (swap high/low bytes)
TCS           ; Transfer C (16-bit A) to S
TCD           ; Transfer C (16-bit A) to D
PHB/PLB       ; Push/Pull Data Bank Register
PHD/PLD       ; Push/Pull Direct Page Register
PHK           ; Push Program Bank Register

Mode Switching¶

CLC           ; Clear carry
XCE           ; Exchange Carry with Emulation flag (enter native mode)
SEC           ; Set carry
XCE           ; Exchange Carry with Emulation flag (enter emulation mode)

Bank Register Management¶

LDA #$01      ; Load bank number
PHA           ; Push to stack
PLB           ; Pull to Data Bank Register

Memory Block Operations¶

MVN src,dst   ; Move memory block (ascending)
MVP src,dst   ; Move memory block (descending)

16-bit Operations¶

REP #$20      ; 16-bit accumulator mode
LDA #$1234    ; Load 16-bit immediate value
ADC #$4321    ; 16-bit addition
SBC #$0001    ; 16-bit subtraction

Long Addressing¶

JML $123456   ; Jump Long (sets Program Bank Register)
JSL $123456   ; Jump to Subroutine Long
RTL           ; Return from Subroutine Long
LDA $123456   ; Load from long address
STA $123456   ; Store to long address

Common Assembly Code Snippets¶

System Initialization¶

    CLC                     ; Clear carry flag
    XCE                     ; Switch to native mode
    REP #$30                ; 16-bit A,X,Y
    LDA #$0000              ; Set direct page to 0
    TCD
    LDX #$01FF              ; Set up stack at $01FF
    TXS
    SEP #$20                ; 8-bit A for I/O operations

Playing a Sound Effect¶

    ; Set frequency for PWM channel 0
    LDA #<440               ; A note (440 Hz) - low byte
    STA $FF20
    LDA #>440               ; A note (440 Hz) - high byte
    STA $FF21

    ; Set duty cycle (50%)
    LDA #$7F
    STA $FF22